An appratus for operating a discharge lamp is generally required for operating a discharge lamp. FIG. 1 shows an example of conventional apparatus for operating discharge lamps. The apparatus for operating discharge lamps has a rectifier 2, a smoothing circuit 3, an inverter circuit 4, an output transformer 11 and a discharge lamp 12. The rectifier 2 is adapted for rectifying a commercial frequency power, such as a 50/60 Hz, 100/200 V power from a commercial power supply 1.
The rectified signal from the rectifier 2 is supplied to the inverter circuit 4 through the smoothing circuit 3. The smoothing circuit 3 removes undesired ripples from the rectified signal. The rectifier 2 is a full-wave rectifier. The smoothing circuit 3 is a voltage-split type smoothing circuit. The full-wave rectifier and the voltage-split type smoothing circuit are well-known in the art, so that detailed descriptions thereof are omitted herein. The output transformer 11 includes a primary winding 11a and a secondry winding 11b.
The inverter circuit 4 includes a series circuit of FETs 5 and 6, a choke 7, a control circuit 8, a capacitor 9, a diode 10 and a series circuit of coupling capacitors 17a and 17b. The series circuit of FETs 5 and 6 provides a switching operation for the rectified voltage from the smoothing circuit 3. The choke 7 has a primary winding 7a and a secondary winding 7b. The series circuit of the FETs 5, 6 and the series circuit of the coupling capacitors 17a, 17b are respectively connected across the rectifier 2. The gates of the FETs 5, 6 are connected to outputs of the control circuit 8 so that they are alternately turned ON or OFF at a high frequency under a control of the control circuit 8. The primary winding 11a of the output transformer 11 is coupled between the coupling node of the FETs 5, 6 and the coupling node of the coupling capacitors 17a and 17b through the choke 7. The secondary winding 11b is connected to the discharge lamp 12. The capacitor 9 is coupled across the primary winding 11a of the output transformer 11.
Thus a high frequency AC current flows through a resonant circuit consisting of the primary winding 7a of the choke 7 and the capacitor 9. An output voltage corresponding to the high frequency AC current is produced on the primary winding 11a of the output transformer 11. The secondary winding 7b of the choke 7 is coupled to inputs of the control circuit 8 through the diode 10. Thus an induced voltage responsive to the high frequency AC current appears on the secondary winding 7b of the choke 7 and then rectified by the diode 10.
A rectified voltage is inputted to the control circuit 8 as a DC power source. The regulated high frequency AC current is outputted to the discharge lamp 12 through the output transformer 11. In the output transformer 11, the primary winding 11a is connected in parallel with the capacitor 9 so that a boosted high frequency AC voltage is generated on the second winding 11b for lighting the discharge lamp 12.
In the above example of the conventional apparatus for discharge lamp, the DC power source required for driving the control circuit 8 of the inverter circuit 4 is generated by the secondary winding 7b of the choke 7.
Further, FIG. 2 is a block diagram showing another example of the conventional apparatus for operating discharge lamp. This example differs from the above in that an additional winding 11c is coupled to the output transformer 11 in place of the secondary winding 7b of the choke 7. In this latter example, the DC power, required for the control circuit 8 is generated at the additional winding 11c of the output transformer 11.
In both these conventional apparatus for operating discharge lamp, the DC power source for the control circuit 8 is obtained from the secondary windings 7b of the choke 7 or the additional winding 11c of the output transformer 11. Therefore, if the high frequency AC current flowing through the choke 7 and the transformer 11 has changed, the DC power source to the control circuit 8 also changes in response to the change and interferes with the stable operation of the control circuit 8.